fix(turns): emit user prompt on the turn event stream (CR-3)

A pure watcher (subscribed but not the sender) couldn't see the user prompt
until the turn sealed: the user message was only persisted at seal and never
entered the live/replayable stream. Add an additive TurnInputEvent
{type:"user-message", conversationId, turnId, text} to the AgentEvent union and
emit it via the broadcast/buffer path as the first event of every turn, so it is
replayed to all subscribers (live + late-join) and on the HTTP path. Persistence
and metrics unchanged; the union widening breaks no exhaustive switch.

- @dispatch/wire 0.5.0->0.6.0; @dispatch/transport-contract 0.7.0->0.8.0 (re-export)
- session-orchestrator: emit user-message at runTurnDetached start; +3 tests,
  3 Wave-1 tests updated (user-message precedes turn-start)
- FE courier: frontend-cr3-user-message-handoff.md

Live-verified vs flash: watcher receives user-message (correct text) as its first
chat.delta before turn-sealed. 894 vitest + transport bun green; tsc -b EXIT 0.

0 files changed, 0 insertions, 0 deletions